1. Field of the Invention
The present invention relates to a surface defect inspector and a method of inspecting a surface defect. More particularly, the present invention relates to a surface defect inspector and a method of inspecting a surface defect, in which a surface defect with a small depth, such as a wrinkle, can be inspected and detected reliably.
2. Description Related to the Prior Art
Cellulose acylate film is polymer film having high performance owing to optically isotropic property, and widely used as protection film of a polarization plate of a liquid crystal display device, an optical compensatory film, and the like. Among various examples, cellulose triacetate film is specifically used.
Cellulose acylate film is produced by solution casting. Polymer or cellulose acylate is dissolved in organic solvent, to prepare polymer solution, which is generally referred to as dope. In a caster, the dope is cast on a support, to form cast film. When the cast film comes to have a self-supporting property with time, polymer film is formed and stripped from the support. A tentering machine is supplied with the polymer film, which is transported while its selvedge portions are clipped for stretching. The polymer film is dried and also oriented in the width direction. An edge slitter is disposed downstream from the tentering machine, and slits away the selvedge portions. Then the polymer film is passed through a drier, and then wound by a winder.
The production of the cellulose acylate film is strictly managed by use of various techniques. However, it is extremely difficult to produce completely defect-free products of the polymer film due to incidental presence of foreign particles and occurrence of wrinkles and unevenness in the thickness of the polymer film. In particular, the occurrence of the wrinkle causes a serious problem for the quality of cellulose acylate film and any types of the polymer film.
There is a known inspection of the wrinkle on the polymer film. A roller of a black color is used. A polymer film 3 is supported on the roller. In FIG. 12A, a pattern of stripes 2 is projected on the polymer film 3 by use of an illuminating light source. When the wrinkle exists on the polymer film 3, a distortion occurs on lines of the stripes 2 in an indicated portion 4 of FIG. 12B. This state is photographed by a camera, to retrieve distinct portions between the obtained image and an image in FIG. 12A. The retrieved image of FIG. 12C is subjected to image processing, to detect finely protruded or recessed pattern of the wrinkle on the surface of the polymer film 3.
A method of inspecting distortion of plates in place of the polymer film is known. JP-A 3-135704 discloses such a method of inspecting reflection distortion or transmission distortion of a glass plate or the like. A test pattern containing bright and dark portions of spots or lines is picked up by use of the glass plate. The distortion is detected by image processing of image data of an image obtained by the image pickup.
Also, inclined wrinkles are likely to occur in the vicinity of the selvedge portions in a section between the tentering machine and edge slitter. The inclined wrinkles are inclined relative of the running of the polymer film 3. As the inclined wrinkles have a small depth, has a small length, and occurs continually. It is impossible to detect the inclined wrinkles even by human eyes, micro meters, laser inspector or the like. In FIG. 13A, stripes 6 are included in a test pattern, and have a smaller interval than that between the stripes 2 of FIG. 12A for detecting the wrinkle. In FIG. 13B, an indicated portion 5 has distortion of lines of the stripes 6. The distortion is so small that inspection of the inclined wrinkles is highly difficult. In addition to the inclined wrinkles, fine wrinkles or other small defects are difficult to find in the above inspecting method.